1. Field of the Invention
The present invention relates to a printing apparatus, and more specifically, to a printing apparatus comprising a device for reading the results of printing performed on a printing medium such as a printing sheet, so that the printing position can be automatically adjusted.
2. Description of the Related Arts
A printing position adjustment technique is known whereby a position relation between respective dots formed by a printing head during forward and backward scans are adjusted to be normal, or whereby a position relation between respective dots formed by a plurality of printing heads are adjusted to be normal. One of specific position adjustment processes, in the case for the forward and backward scans, is such that predetermined patterns each composed of respective dots formed during the forward and backward scans are respectively printed on a plurality of printing sheets at different timings of dot formation, and are read by an optical measurement device. Then, based on the reading results, a pattern that realizes the best position relation of the respective dots is selected, and the formation timing for this pattern is employed to designate a dot formation timing for the printing process. The position adjustment for a plurality of printing heads can be performed substantially in the same manner. Hereinafter, this position adjustment process is called a registration adjustment, or an automatic registration adjustment, as opposed to a user performed registration adjustment. An example automatic registration adjustment process is described in Japanese Patent Application Laid-open No. 11-291477 (1999).
According to Japanese Patent Application Laid-open No. 11-291477 (1999), a sensor constituting an optical measurement device is located at a downstream side of a printing head in a direction in which a printing sheet is fed, and therefore a pattern printed for the registration adjustment can be read while the printing sheet is being fed in the same direction as for usual printing.
There is another apparatus wherein, in order to reduce the apparatus size, a sensor functioning as an optical measurement device is located at an upstream side, in the feeding direction, of a printing head. This apparatus can not read a registration adjustment pattern in a single feeding direction, unlike the apparatus in Japanese Patent Application Laid-open No. 11-291477 (1999). So in this case, it is viewed as a printing sheet bearing a registration adjustment pattern is fed in the reverse direction to permit the sensor to read the pattern. However, a defect, such as a so-called paper jam, may occur when a printing sheet is reversely fed.
More specifically, for a general printing apparatus, an automatic sheet feeder separates one printing sheet and supplies the separated sheet. Then, a feeding mechanism feeds the printing sheet to a printing area for a printing head and successively feeds the printing sheet in the same direction so that a printing operation is performed for the printing sheet. For this configuration, when the printing sheet is reversely fed, the print sheet is fed along a feeding path extending from the automatic sheet feeder to the printing area. As a result, the edge of the printing sheet may be caught by a paper detection sensor located along the feeding path, a separation unit in the automatic sheet feeder, guides forming the feeding path, or the like, so holding of the print sheet or a paper jam may occur, which prevents the registration adjustment pattern printed by the printing head from being measured correctly.
Especially when a large proportion of the area of a printing sheet is required for the printing of registration patterns, the distance in the reverse feeding direction must be extended, and accordingly, the probability a paper jam or the like will occur is increased. For example, to print on a printing sheet multiple patterns for which the dot formation timings differ, a very large printing area is required. In addition, these patterns are required for the individual printing heads provided in accordance with the types of printing materials, such as ink. Furthermore, when a large printing area on a printing sheet is available for the printing of registration adjustment patterns, the degree of freedom with which patterns are positioned on the printing sheet can be increased. When, however, a large area is not available for the printing of registration adjustment patterns, and when, to avoid the paper jam or the like, a printing sheet is reversely fed only a short distance, multiple registration adjustment patterns, which should be printed on a single printing sheet, must be printed on a plurality of printing sheets.
On the other hand, according to one proposal advanced for avoiding the reverse feeding that may cause the paper jam or the like, a printing sheet on which the registration patterns are printed is again loaded into the automatic sheet feeder and fed to the measurement area for the sensor. In this case, however, the reloading of the printing sheet must be performed by the user.